In manufacture of silicon single crystals, stably manufacturing silicon single crystals having desired quality is important to prevent production loss and enhance a product yield. In particular, a problem lies in how grown-in defects in a silicon single crystal can be reduced and how low-defect silicon single crystal can be stably manufactured due to high integration of semiconductor devices and attendant progress of miniaturization in recent years.
It is known that the grown-in defect is determined based on a temperature gradient of a crystal at a growth interface and a growth rate of the silicon single crystal, and the temperature gradient of the crystal at the growth interface must be highly accurately controlled.
To control the temperature gradient of the crystal at the growth interface, a cylindrical heat insulating component which surrounds a periphery of a silicon single crystal grown above a melt surface and blocks radiant heat is provided in conventional examples. Consequently, a crystal temperature gradient when the crystal has a high temperature can be increased, and a defect-free crystal can be rapidly provided.
As described above, in the silicon single crystal manufacturing apparatus having the heat insulating component provided therein, to accurately control the crystal temperature gradient at the growth interface, a gap between the melt surface and a lower end of the heat insulating component must be accurately controlled to be a predetermined gap.
At the time of growing a silicon single crystal, a silicon melt contained in a crucible reduces with growth of the silicon single crystal, and a melt surface position descends. Thus, there has been conventionally adopted a method for controlling a melt surface position by estimating an amount of descent of the melt surface position in accordance with growth of a silicon single crystal, issuing an ascending instruction to a crucible holding shaft in correspondence with an estimated value, and ascending a crucible position to prevent descent of the melt surface position so that the melt surface position can be maintained constant at a predetermined position.
However, with an increase in diameter of a crucible shape associated with an increase in crystal diameter, the melt surface position largely changes due to a variation in wall thickness of the crucible and deformation and expansion of the crucible which occur during an operation. Thus, it is difficult to accurately control the melt surface position to be maintained at the predetermined position by solely performing such ascent control over a crucible position in correspondence with an estimated value as described above.
Thus, there has been adopted such a method as disclosed in, e.g., Patent Literature 1 or Patent Literature 2, by which a CCD camera configured to measure a melt surface position from the outside of a furnace is provided to an outer portion of a chamber and the melt surface position is accurately controlled to a fixed position based on a measurement result from an image provided by the CCD camera.
Specifically, Patent Literature 1 discloses a method for imaging a reference reflector disposed at a lower end of a heat insulating component present above a silicon melt and the reference reflector reflected on the melt surface which is like a specular surface by using an optical apparatus such as a CCD camera, and measuring a melt surface portion from this video.
Furthermore, Patent Literature 3 discloses a method for comparing a crystal diameter measured by first diameter measuring means using a CCD camera installed at an arbitrary angle to a crystal with a crystal diameter measured by second diameter measuring means using two CCD cameras juxtaposed to both ends of the crystal, and calculating a melt surface position from a difference between the first crystal diameter and the second crystal diameter.
As a method for setting a melt surface position acquired by each of such measuring methods to a desired position, there is adopted a method for calculating a deviation of a current melt surface position from a measured melt surface position and a desired melt surface position, and correcting a crucible ascending rate in correspondence with the calculated deviation so that the melt surface position can be controlled to a desired position.